Differential field effect sensing elements are known; they are suitable to determine the speed of rotary elements, or, if desired, the angular position of a shaft, or other rotating element with respect to a reference position which, for example, may be the position of the field effect element as such. Field effect elements are magnetically responsive semi-conductor elements which, usually, have two field plates located on the soft iron pole shoe. A permanent magnet is located at the bottom side of the sensing head, which provides a magnetic bias. If a body of soft magnetic material approaches the facing side of the sensor, the field plates will change their resistance. Such a body may, for example, be the tooth of a gear; if the element is used to determine the speed, or angular position of a crank shaft of an internal combustion engine, and hence of the piston within a cylinder, the gear may, suitably, be the starter gear coupled to the fly wheel of the internal combustion (IC) engine. A detailed description of magnetic field effect sensors is found in the Siemens Data Book 76/77, chapter on "Magnetically Responsive Semi-Conductors" (page 160).
Magnetic field effect sensors are highly temperature sensitive. The field plates are applied to a substrate by adhesives, and high temperatures may interfere with the adhesive quality of the adhesive material, and additionally destroy the meander shape of the field plate resistance elements. Differential field plate sensors, additionally, require high magnetic bias. This, in turn, requires a high quality, and hence expensive, permanent magnet to provide the necessary magnetic bias field.